1. Field of the invention
This invention relates to a capacitance type weight sensor.
2. Description of the Prior Art
An example of conventional capacitance type weight sensors as used in a weighing instrument such as a capacitance type electronic digital weighing instrument is disclosed in the official gazette of Japanese Patent Application Laid-open No. 55-147317. As schematically shown in FIG. 5 of the accompanying drawings, the conventional capacitance type weight sensor comprises a Roberval mechanism having a stationary strut 1 and a movable strut 2 each formed in the shape of letter U and a pair of beams 3 and 4 of leaf springs connecting the top parts and bottom parts of the stationary and movable struts 1 and 2, a pair of electrodes 7 and 8 fixed to the vertical surfaces of the respective struts 1 and 2 of the Roberval mechanism through insulator members 5 and 6, a base 9 fixedly supporting the vertical surface of the stationary strut 1, and a bowl 10 fixed to the vertical surface of the movable strut 2.
In such a capacitance type weight sensor utilizing a Roberval mechanism, a load applied to the bowl 10 can be detected by conventing the deformation magnitudes of the beams 3 and 4 which flex in proportion to the load into a variation in the capacitance between the electrodes 7 and 8.
However, with the conventional Roberval mechanism mentioned above, the leaf springs functioning as the beams are deformed at the junction points between the two struts 1 and 2 and the two beams 3 and 4 constituting the Roberval mechanism, the deformations of the leaf springs cause the junctions to shift, and the shifts form the factor of weighing errors. Further, since the fixing references of the electrodes and the references of the fixation to the base and the fixation to the bowl receptacle are the vertical surfaces of the strut portions, the deformations of the strut portions form the factor of weighing errors. Another disadvantage is that, since the whole beam undergoes a twist due to a lateral load (a load which deviates from the center line of the beam in the widthwise direction thereof), the inclination of the movable strut ascribable to the lateral load is great, to incline the movable electrode and to break the parallelism of the two electrodes, resulting in the factor of measurement errors. The inclination of the movable strut attributed to the lateral load increases in inverse proportion to the height of the strut portions, and forms an obstacle to the thinning of a product.
Moreover, in such a capacitance type weight sensor, deviations in the parallelism of the electrodes and in the heights of the struts, and the inclinations of the struts form great factors for errors ascribable to biased loads. It exerts a great influence on the cost of fabrication to work the individual constituent components at high precision, and it is not easy to fix the two electrodes to the struts in parallel, so that the errors ascribable to the biased loads deviate greatly. In Japanese Patent Application Laid-open No. 58-176525, therefore, it has been proposed that the parts of the electrodes to be fixed to the struts are improved, and the parallelism between the whole electrode plates is adjusted in an assembling operation, thereby to lessen the biased load errors. However, the deviation of the parallelism of the whole electrode plates is greatly influential on the biased load errors, and the alteration of an angle in the adjustment must be very slight, so that a skill is required for the adjustment. The timing of the locking of a fixation screw (with a locking agent or the like) is also a problem.
Therefore, an object of this invention is to eliminate the disadvantages mentioned above and provide a capacitance type weight sensor which is thin and with a high accuracy.
Another object of this invention is to provide a capacitance type weight sensor wherein fine adjustments for load detecting accuracy can be easily made.